1. Field of the Invention
This invention relates generally to record disks, and more particularly is directed to an improved record disk which may be conveniently mass produced and which has a spiral groove in its surface with the depth of such groove varying therealong for representing the recorded information or signals.
2. Description of the Prior Art
The disk recording of audio signals is well known, and involves the use of a lacquer disk for the original recording. Such original lacquer disk, usually constituted by an aluminum base plate with a coating of acetate plastic thereon, is placed on a recording turntable which is rotated at a suitably slow standard speed for audio recording, for example, 331/3, 45 or 78 r.p.m. During the rotation of the lacquer disk, a cutter including an electromechanical transducer and a cutting stylus is driven in the radial direction of the turntable by means of a lead screw so that the cutting stylus cuts a spiral groove of constant pitch in the acetate plastic coating. As the spiral groove is being cut, electrical signals corresponding to the audio or sound information to be recorded are applied to a driving coil of the cutter transducer for effecting corresponding vibrations of the cutting stylus with the result that undulations are formed in the spiral groove to represent the recorded information or signals. In mass producing phonograph record disks from such original lacquer disk, the latter is metallized and then electroplated, whereupon the plating is separated from the original lacquer disk and reenforced by backing with a solid metal plate to produce the so-called master. Thereafter, by well-known steps, a mother and then a stamper are produced from the master, whereupon, the stamper is employed in a hydraulic press for molding successive phonograph records from preforms or biscuits of thermoplastic material, with each such phonograph record having a modulated spiral groove in its surface corresponding to the groove formed in the respective original lacquer disk.
When phonograph records are produced in the conventional manner described above, the speed of rotation of the original lacquer disk during the cutting of the spiral groove therein is relatively slow so that an adequate playing time can be achieved even if the pitch of the spiral groove is large enought to ensure that adjacent turns of the spiral groove will not interfere or extend into each other whether the recorded signals are represented by lateral undulations in the groove or variations in the depth of the latter.
Recently, it has been proposed to record video signals as variations in the depth of a spiral groove formed in the surface of a record disk by a method similar to that described above for the recording of audio signals. If the resulting video record disk is to have one frame of the video signals recorded in each turn of its spiral groove, it will be necessary, during reproducing of the recorded video signals, to rotate the video record disk at a high speed, for example, 1800 r.p.m. in the case of NTSC video signals, from which it follows that the spiral groove of the record disk must have 1800 turns for each minute of its playing time. Therefore, if a video record disk of reasonable diameter is to be provided with a substantial playing time, for example of 30 minutes, the pitch of the spiral groove has to be very small, for example, on the order of 3 to 4 microns. However, when the original recording of video signals is produced by the mechanical cutting of a spiral groove in the original lacquer disk, as described above, the cross-sectional configuration of the groove is constant, for example, is V-shaped with a constant apex angle, as determined by the shape of the cutting stylus. Therefore, as the depth of the groove is varied to represent the recorded video signals, the width of the groove at the surface of the record disk is correspondingly varied. By reason of the small pitch of the spiral groove required in recording video signals, the width of the groove of constant V-shaped cross-sectional configuration will exceed such small pitch, at least at the locations of maximum depth of the groove, with the result that adjacent turns of the spiral groove will interfere or extend into each other. In other words, at least at the locations of maximum depth of the groove, the adjacent side surfaces of successive turns of the spiral groove will intersect below the surface of the original lacquer disk in which the groove is being cut with the result that the height of the wall separating successive turns will vary. Moreover, it has been found that such variation of the height of the wall separating successive turns of the spiral groove in the video record disk undesirably affects the reproduction of the recorded video signals in that, during reproduction of the signals recorded as variations in the depth of the groove in one turn thereof, the reproduced signal may include cross-talk components from the next adjacent turns of the groove.